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href="/mysql/06.html" data-v-59eaa146 data-v-531573d4><p class="link-text" data-v-531573d4>06 【多表查询】</p></a><a class="link" href="/mysql/07.html" data-v-59eaa146 data-v-531573d4><p class="link-text" data-v-531573d4>07 【单行函数】</p></a><a class="link" href="/mysql/08.html" data-v-59eaa146 data-v-531573d4><p class="link-text" data-v-531573d4>08 【聚合函数与分组查询】</p></a><a class="link" href="/mysql/09.html" data-v-59eaa146 data-v-531573d4><p class="link-text" data-v-531573d4>09 【子查询】</p></a><a class="link" href="/mysql/10.html" data-v-59eaa146 data-v-531573d4><p class="link-text" data-v-531573d4>10 【创建和管理表】</p></a><a class="link" href="/mysql/11.html" data-v-59eaa146 data-v-531573d4><p class="link-text" data-v-531573d4>11 【数据处理之增删改】</p></a><!--]--></section></div><!--]--><!--[--><!--]--></nav></aside><div id="VPContent" class="VPContent has-sidebar" data-v-23306c50 data-v-790151bc><div class="VPContentDoc has-aside has-sidebar" data-v-790151bc data-v-4fe9b7bd><div class="container" data-v-4fe9b7bd><div class="aside" data-v-4fe9b7bd><div class="aside-container" data-v-4fe9b7bd><!--[--><!--]--><div class="VPContentDocOutline" data-v-4fe9b7bd data-v-aa0e2252><div class="outline-marker" data-v-aa0e2252></div><div class="outline-title" data-v-aa0e2252>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-aa0e2252><span id="doc-outline-aria-label" class="visually-hidden" data-v-aa0e2252>Table of Contents for current page</span><ul class="root" data-v-aa0e2252><!--[--><li style="" data-v-aa0e2252><a class="outline-link" href="#_1-sql分类" data-v-aa0e2252>1.SQL分类</a><!----></li><li style="" data-v-aa0e2252><a class="outline-link" href="#_2-sql语言的规则与规范" data-v-aa0e2252>2.SQL语言的规则与规范</a><!----></li><li style="" data-v-aa0e2252><a class="outline-link" href="#_3-基本的select语句" data-v-aa0e2252>3.基本的SELECT语句</a><!----></li><li style="" data-v-aa0e2252><a class="outline-link" href="#_4-显示表结构" data-v-aa0e2252>4. 显示表结构</a><!----></li><li style="" data-v-aa0e2252><a class="outline-link" href="#_5-过滤数据" data-v-aa0e2252>5. 过滤数据</a><!----></li><li style="" data-v-aa0e2252><a class="outline-link" href="#_6-课后练习" data-v-aa0e2252>6.课后练习</a><!----></li><!--]--></ul></nav></div><!--[--><!--[--><!--[--><div><a class="sponsors-aside-text" href="/">联系客服</a><img src="/images/qrcode.png"></div><!--]--><!--]--><!--]--><div class="VPCarbonAds" data-v-4fe9b7bd><img src="/logo.svg" alt="" srcset=""></div><!--[--><!--[--><!--[--><h1 class="tagline" data-v-3916b2f0><span class="accent" data-v-3916b2f0>Bin</span><br data-v-3916b2f0>QQ - 1974892005 </h1><!--]--><!--]--><!--]--></div></div><div class="content" data-v-4fe9b7bd><!--[--><!--]--><main data-v-4fe9b7bd><div style="position:relative;" class="vt-doc mysql" data-v-4fe9b7bd><div><h1 id="_03-【基本的select语句】" tabindex="-1">03 【基本的SELECT语句】 <a class="header-anchor" href="#_03-【基本的select语句】" aria-hidden="true">#</a></h1><h2 id="_1-sql分类" tabindex="-1">1.SQL分类 <a class="header-anchor" href="#_1-sql分类" aria-hidden="true">#</a></h2><p>SQL语言在功能上主要分为如下3大类：</p><ul><li><strong>DDL（Data Definition Languages、数据定义语言）</strong>，这些语句定义了不同的数据库、表、视图、索引等数据库对象，还可以用来创建、删除、修改数据库和数据表的结构。 <ul><li>主要的语句关键字包括<code>CREATE</code>、<code>DROP</code>、<code>ALTER</code>、<code>RENAME</code>、<code>TRUNCATE</code>等。</li></ul></li><li><strong>DML（Data Manipulation Language、数据操作语言）</strong>，用于添加、删除、更新和查询数据库记录，并检查数据完整性。 <ul><li>主要的语句关键字包括<code>INSERT</code>、<code>DELETE</code>、<code>UPDATE</code>、<code>SELECT</code>等。</li><li><strong>SELECT是SQL语言的基础，最为重要。</strong></li></ul></li><li><strong>DCL（Data Control Language、数据控制语言）</strong>，用于定义数据库、表、字段、用户的访问权限和安全级别。 <ul><li>主要的语句关键字包括<code>GRANT</code>、<code>REVOKE</code>、<code>COMMIT</code>、<code>ROLLBACK</code>、<code>SAVEPOINT</code>等。</li></ul></li></ul><p>注：因为查询语句使用的非常的频繁，所以很多人把查询语句单拎出来一类：<strong>DQL（数据查询语言）</strong>。还有单独将<code>COMMIT</code>、<code>ROLLBACK</code> 取出来称为<strong>TCL （Transaction Control Language，事务控制语言）</strong>。</p><h2 id="_2-sql语言的规则与规范" tabindex="-1">2.SQL语言的规则与规范 <a class="header-anchor" href="#_2-sql语言的规则与规范" aria-hidden="true">#</a></h2><h3 id="_2-1-基本规则" tabindex="-1">2.1 基本规则 <a class="header-anchor" href="#_2-1-基本规则" aria-hidden="true">#</a></h3><ul><li>SQL 可以写在一行或者多行。为了提高可读性，各子句分行写，必要时使用缩进</li><li>每条命令以 ; 或 \g 或 \G 结束</li><li>关键字不能被缩写也不能分行</li><li>关于标点符号 <ul><li>必须保证所有的()、单引号、双引号是成对结束的</li><li>必须使用英文状态下的半角输入方式</li><li>字符串型和日期时间类型的数据可以使用单引号 &#39; &#39; 表示</li><li>列的别名，尽量使用双引号 &quot; &quot; ，切记不要使用单引号 ，而且不建议省略as</li></ul></li></ul><h3 id="_2-2-sql大小写规范-（建议遵守）" tabindex="-1">2.2 SQL大小写规范 （建议遵守） <a class="header-anchor" href="#_2-2-sql大小写规范-（建议遵守）" aria-hidden="true">#</a></h3><ul><li><strong>MySQL 在 Windows 环境下是大小写不敏感的</strong></li><li>MySQL 在 Linux 环境下是大小写敏感的 <ul><li>数据库名、表名、表的别名、变量名是严格区分大小写的</li><li>关键字、函数名、列名(或字段名)、列的别名(字段的别名) 是忽略大小写的。</li></ul></li><li><code>推荐采用统一的书写规范：</code><ul><li>数据库名、表名、表别名、字段名、字段别名等都小写</li><li>SQL 关键字、函数名、绑定变量等都大写</li></ul></li></ul><h3 id="_2-3-注释" tabindex="-1">2.3 注释 <a class="header-anchor" href="#_2-3-注释" aria-hidden="true">#</a></h3><p>可以使用如下格式的注释结构</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">单行注释：#注释文字(MySQL特有的方式)</span></span>
<span class="line"><span style="color:#A6ACCD;">单行注释：</span><span style="color:#676E95;font-style:italic;">-- 注释文字(--后面必须包含一个空格。)</span></span>
<span class="line"><span style="color:#A6ACCD;">多行注释：</span><span style="color:#676E95;font-style:italic;">/* 注释文字  */</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="_2-4-命名规则（暂时了解）" tabindex="-1">2.4 命名规则（暂时了解） <a class="header-anchor" href="#_2-4-命名规则（暂时了解）" aria-hidden="true">#</a></h3><ul><li>数据库、表名不得超过30个字符，变量名限制为29个</li><li>必须只能包含 A–Z, a–z, 0–9, _共63个字符</li><li>数据库名、表名、字段名等对象名中间不要包含空格</li><li>同一个MySQL软件中，数据库不能同名；同一个库中，表不能重名；同一个表中，字段不能重名</li><li><strong>必须保证你的字段没有和保留字、数据库系统或常用方法冲突。如果坚持使用，请在SQL语句中使用`（着重号）引起来</strong></li><li>保持字段名和类型的一致性，在命名字段并为其指定数据类型的时候一定要保证一致性。假如数据类型在一个表里是整数，那在另一个表里可就别变成字符型了</li></ul><p>举例：</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">#以下两句是一样的，不区分大小写</span></span>
<span class="line"><span style="color:#A6ACCD;">show databases;</span></span>
<span class="line"><span style="color:#A6ACCD;">SHOW DATABASES;</span></span>
<span class="line"><span style="color:#A6ACCD;"> </span></span>
<span class="line"><span style="color:#A6ACCD;">#创建表格</span></span>
<span class="line"><span style="color:#A6ACCD;">#</span><span style="color:#F78C6C;">create</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">table</span><span style="color:#A6ACCD;"> student info(...); #表名错误，因为表名有空格</span></span>
<span class="line"><span style="color:#F78C6C;">create</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">table</span><span style="color:#A6ACCD;"> student_info(...); </span></span>
<span class="line"><span style="color:#A6ACCD;"> </span></span>
<span class="line"><span style="color:#A6ACCD;">#其中order使用</span><span style="color:#89DDFF;">``</span><span style="color:#A6ACCD;">飘号，因为order和系统关键字或系统函数名等预定义标识符重名了</span></span>
<span class="line"><span style="color:#F78C6C;">CREATE</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">TABLE</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">order</span><span style="color:#89DDFF;">`</span><span style="color:#A6ACCD;">(</span></span>
<span class="line"><span style="color:#A6ACCD;">    id </span><span style="color:#F78C6C;">INT</span><span style="color:#A6ACCD;">,</span></span>
<span class="line"><span style="color:#A6ACCD;">    lname </span><span style="color:#F78C6C;">VARCHAR</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">20</span><span style="color:#A6ACCD;">)</span></span>
<span class="line"><span style="color:#A6ACCD;">);</span></span>
<span class="line"><span style="color:#A6ACCD;"> </span></span>
<span class="line"><span style="color:#A6ACCD;"># 正确</span></span>
<span class="line"><span style="color:#F78C6C;">select</span><span style="color:#A6ACCD;"> id </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">编号</span><span style="color:#89DDFF;">&quot;</span><span style="color:#A6ACCD;">, </span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">name</span><span style="color:#89DDFF;">`</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">姓名</span><span style="color:#89DDFF;">&quot;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">from</span><span style="color:#A6ACCD;"> t_stu; #起别名时，as都可以省略</span></span>
<span class="line"><span style="color:#A6ACCD;"># 正确</span></span>
<span class="line"><span style="color:#F78C6C;">select</span><span style="color:#A6ACCD;"> id </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> 编号, </span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">name</span><span style="color:#89DDFF;">`</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> 姓名 </span><span style="color:#F78C6C;">from</span><span style="color:#A6ACCD;"> t_stu; #如果字段别名中没有空格，那么可以省略</span><span style="color:#89DDFF;">&quot;&quot;</span></span>
<span class="line"><span style="color:#A6ACCD;">#错误</span></span>
<span class="line"><span style="color:#F78C6C;">select</span><span style="color:#A6ACCD;"> id </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> 编 号, </span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">name</span><span style="color:#89DDFF;">`</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> 姓 名 </span><span style="color:#F78C6C;">from</span><span style="color:#A6ACCD;"> t_stu; #错误，如果字段别名中有空格，那么不能省略</span><span style="color:#89DDFF;">&quot;&quot;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><h3 id="_2-5-数据导入指令" tabindex="-1">2.5 数据导入指令 <a class="header-anchor" href="#_2-5-数据导入指令" aria-hidden="true">#</a></h3><p>在命令行客户端登录mysql，使用source指令导入</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">mysql</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> source d:\mysqldb.sql</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">mysql</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">desc</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------------+-------------+------+-----+---------+-------+</span></span>
<span class="line"><span style="color:#A6ACCD;">| Field          | </span><span style="color:#F78C6C;">Type</span><span style="color:#A6ACCD;">        | </span><span style="color:#F78C6C;">Null</span><span style="color:#A6ACCD;"> | </span><span style="color:#F78C6C;">Key</span><span style="color:#A6ACCD;"> | </span><span style="color:#C792EA;">Default</span><span style="color:#A6ACCD;"> | Extra |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------------+-------------+------+-----+---------+-------+</span></span>
<span class="line"><span style="color:#A6ACCD;">| employee_id    | </span><span style="color:#F78C6C;">int</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">6</span><span style="color:#A6ACCD;">)      | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   | PRI | </span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">       |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| first_name     | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">20</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| last_name      | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">25</span><span style="color:#A6ACCD;">) | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| email          | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">25</span><span style="color:#A6ACCD;">) | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   | UNI | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| phone_number   | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">20</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| hire_date      | </span><span style="color:#F78C6C;">date</span><span style="color:#A6ACCD;">        | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| job_id         | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">10</span><span style="color:#A6ACCD;">) | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   | MUL | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| salary         | double(</span><span style="color:#F78C6C;">8</span><span style="color:#A6ACCD;">,</span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| commission_pct | double(</span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">,</span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| manager_id     | </span><span style="color:#F78C6C;">int</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">6</span><span style="color:#A6ACCD;">)      | YES  | MUL | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| department_id  | </span><span style="color:#F78C6C;">int</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">4</span><span style="color:#A6ACCD;">)      | YES  | MUL | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------------+-------------+------+-----+---------+-------+</span></span>
<span class="line"><span style="color:#F78C6C;">11</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">rows</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">in</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">set</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">.</span><span style="color:#F78C6C;">00</span><span style="color:#A6ACCD;"> sec)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><h2 id="_3-基本的select语句" tabindex="-1">3.基本的SELECT语句 <a class="header-anchor" href="#_3-基本的select语句" aria-hidden="true">#</a></h2><h3 id="_3-1-select-from" tabindex="-1">3.1 SELECT ... FROM <a class="header-anchor" href="#_3-1-select-from" aria-hidden="true">#</a></h3><ul><li>语法：</li></ul><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;">   标识选择哪些列 </span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">	 标识从哪个表中选择</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><ul><li>选择全部列：</li></ul><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   departments;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://i0.hdslb.com/bfs/album/1d2dfd6299a889354e954ad4517fae248fea4caf.png" alt=""></p><blockquote><p>一般情况下，除非需要使用表中所有的字段数据，最好不要使用通配符‘*’。使用通配符虽然可以节省输入查询语句的时间，但是获取不需要的列数据通常会降低查询和所使用的应用程序的效率。通配符的优势是，当不知道所需要的列的名称时，可以通过它获取它们。</p><p>在生产环境下，不推荐你直接使用<code>SELECT *</code>进行查询。</p></blockquote><ul><li>选择特定的列：</li></ul><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> department_id, location_id</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   departments;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><blockquote><p>MySQL中的SQL语句是不区分大小写的，因此SELECT和select的作用是相同的，但是，许多开发人员习惯将关键字大写、数据列和表名小写，读者也应该养成一个良好的编程习惯，这样写出来的代码更容易阅读和维护。</p></blockquote><h3 id="_3-2-列的别名" tabindex="-1">3.2 列的别名 <a class="header-anchor" href="#_3-2-列的别名" aria-hidden="true">#</a></h3><ul><li><p>重命名一个列</p></li><li><p>便于计算</p></li><li><p>紧跟列名，也可以<strong>在列名和别名之间加入关键字AS，<code>别名使用双引号(不能使用单引号)</code></strong>，以便在别名中包含空格或特殊的字符并区分大小写。</p></li><li><p>AS 可以省略</p></li><li><p>建议别名简短，见名知意</p></li><li><p>举例</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> last_name </span><span style="color:#F78C6C;">AS</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">name</span><span style="color:#A6ACCD;">, commission_pct comm</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div></li></ul><p><img src="https://i0.hdslb.com/bfs/album/463c03536c07786a91f1d0b75169408f2e037584.png" alt=""></p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> last_name </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Name</span><span style="color:#89DDFF;">&quot;</span><span style="color:#A6ACCD;">, salary</span><span style="color:#89DDFF;">*</span><span style="color:#F78C6C;">12</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Annual Salary</span><span style="color:#89DDFF;">&quot;</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://i0.hdslb.com/bfs/album/6c019987e827f018ccd7f54ab58b26c287efc5e2.png" alt=""></p><h3 id="_3-3-去除重复行" tabindex="-1">3.3 去除重复行 <a class="header-anchor" href="#_3-3-去除重复行" aria-hidden="true">#</a></h3><p>默认情况下，查询会返回全部行，包括重复行。</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> department_id</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://i0.hdslb.com/bfs/album/f7b92eef771141bee11ccfd51bdb677f151bbca3.png" alt=""></p><p><strong>在SELECT语句中使用关键字DISTINCT去除重复行</strong></p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT DISTINCT</span><span style="color:#A6ACCD;"> department_id</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://i0.hdslb.com/bfs/album/e376c1ab86fe5521739f32de0e649bf8ca68436b.png" alt=""></p><p>针对于：</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT DISTINCT</span><span style="color:#A6ACCD;"> department_id,salary </span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这里有两点需要注意：</p><ol><li>DISTINCT 需要放到所有列名的前面，如果写成<code>SELECT salary, DISTINCT department_id FROM employees</code>会报错。</li><li>DISTINCT 其实是<code>对后面所有列名的组合进行去重</code>（也就是说 department_id 和salary被看成一个整体，其他记录的department_id 和salary 必须等于前面出现的才可以去重），你能看到最后的结果是 74 条，因为这 74 个部门id不同，都有 salary 这个属性值。如果你想要看都有哪些不同的部门（department_id），只需要写<code>DISTINCT department_id</code>即可，后面不需要再加其他的列名了。</li></ol><h3 id="_3-4-空值参与运算" tabindex="-1">3.4 空值参与运算 <a class="header-anchor" href="#_3-4-空值参与运算" aria-hidden="true">#</a></h3><p>所有运算符或列值遇到null值，运算的结果都为null</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> employee_id,salary,commission_pct,</span></span>
<span class="line"><span style="color:#F78C6C;">12</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> salary </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> commission_pct) </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">annual_sal</span><span style="color:#89DDFF;">&quot;</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>这里你一定要注意，在 MySQL 里面， 空值null不等于空字符串。一个空字符串的长度是 0，而一个空值的长度是空。而且，在 MySQL 里面，空值是占用空间的。</p><p>补充：null 不等于 0，不等于 &#39; &#39;，也不等于 &#39;null&#39;</p><h3 id="_3-5-着重号" tabindex="-1">3.5 着重号 <a class="header-anchor" href="#_3-5-着重号" aria-hidden="true">#</a></h3><ul><li>错误的</li></ul><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">mysql</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> ORDER;</span></span>
<span class="line"><span style="color:#A6ACCD;">ERROR </span><span style="color:#F78C6C;">1064</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">42000</span><span style="color:#A6ACCD;">): You have an error </span><span style="color:#F78C6C;">in</span><span style="color:#A6ACCD;"> your </span><span style="color:#F78C6C;">SQL</span><span style="color:#A6ACCD;"> syntax; </span><span style="color:#C792EA;">check</span><span style="color:#A6ACCD;"> the </span><span style="color:#F78C6C;">manual</span><span style="color:#A6ACCD;"> that corresponds </span><span style="color:#F78C6C;">to</span><span style="color:#A6ACCD;"> your MySQL </span><span style="color:#F78C6C;">server</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">version</span><span style="color:#A6ACCD;"> for the </span><span style="color:#82AAFF;">right</span><span style="color:#A6ACCD;"> syntax </span><span style="color:#F78C6C;">to</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">use</span><span style="color:#A6ACCD;"> near </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">ORDER</span><span style="color:#89DDFF;">&#39;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">at</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">line</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><ul><li>正确的</li></ul><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">mysql</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">ORDER</span><span style="color:#89DDFF;">`</span><span style="color:#A6ACCD;">;</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------+------------+</span></span>
<span class="line"><span style="color:#A6ACCD;">| order_id | order_name |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------+------------+</span></span>
<span class="line"><span style="color:#A6ACCD;">|        </span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;"> | shkstart   |</span></span>
<span class="line"><span style="color:#A6ACCD;">|        </span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;"> | tomcat     |</span></span>
<span class="line"><span style="color:#A6ACCD;">|        </span><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;"> | dubbo      |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------+------------+</span></span>
<span class="line"><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">rows</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">in</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">set</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">.</span><span style="color:#F78C6C;">00</span><span style="color:#A6ACCD;"> sec)</span></span>
<span class="line"><span style="color:#A6ACCD;"> </span></span>
<span class="line"><span style="color:#A6ACCD;">mysql</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">order</span><span style="color:#89DDFF;">`</span><span style="color:#A6ACCD;">;</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------+------------+</span></span>
<span class="line"><span style="color:#A6ACCD;">| order_id | order_name |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------+------------+</span></span>
<span class="line"><span style="color:#A6ACCD;">|        </span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;"> | shkstart   |</span></span>
<span class="line"><span style="color:#A6ACCD;">|        </span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;"> | tomcat     |</span></span>
<span class="line"><span style="color:#A6ACCD;">|        </span><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;"> | dubbo      |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------+------------+</span></span>
<span class="line"><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">rows</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">in</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">set</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">.</span><span style="color:#F78C6C;">00</span><span style="color:#A6ACCD;"> sec)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><ul><li>结论</li></ul><p>我们需要保证表中的字段、表名等没有和保留字、数据库系统或常用方法冲突。如果真的相同，请在SQL语句中使用一对``（着重号）引起来。</p><h3 id="_3-6-查询常数" tabindex="-1">3.6 查询常数 <a class="header-anchor" href="#_3-6-查询常数" aria-hidden="true">#</a></h3><p>SELECT 查询还可以对常数进行查询。对的，就是在 SELECT 查询结果中增加一列固定的常数列。这列的取值是我们指定的，而不是从数据表中动态取出的。</p><p>你可能会问为什么我们还要对常数进行查询呢？</p><p>SQL 中的 SELECT 语法的确提供了这个功能，一般来说我们只从一个表中查询数据，通常不需要增加一个固定的常数列，但如果我们想整合不同的数据源，用常数列作为这个表的标记，就需要查询常数。</p><p>比如说，我们想对 employees 数据表中的员工姓名进行查询，同时增加一列字段<code>corporation</code>，这个字段固定值为“尚硅谷”，可以这样写：</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">尚硅谷</span><span style="color:#89DDFF;">&#39;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">as</span><span style="color:#A6ACCD;"> corporation, last_name </span><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><img src="https://i0.hdslb.com/bfs/album/24ea100148c6461e7e2c51f072d5031fde60002e.png" alt=""></p><h2 id="_4-显示表结构" tabindex="-1">4. 显示表结构 <a class="header-anchor" href="#_4-显示表结构" aria-hidden="true">#</a></h2><p>使用DESCRIBE 或 DESC 命令，表示表结构。</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">DESCRIBE employees;</span></span>
<span class="line"><span style="color:#A6ACCD;">或</span></span>
<span class="line"><span style="color:#F78C6C;">DESC</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">mysql</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">desc</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------------+-------------+------+-----+---------+-------+</span></span>
<span class="line"><span style="color:#A6ACCD;">| Field          | </span><span style="color:#F78C6C;">Type</span><span style="color:#A6ACCD;">        | </span><span style="color:#F78C6C;">Null</span><span style="color:#A6ACCD;"> | </span><span style="color:#F78C6C;">Key</span><span style="color:#A6ACCD;"> | </span><span style="color:#C792EA;">Default</span><span style="color:#A6ACCD;"> | Extra |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------------+-------------+------+-----+---------+-------+</span></span>
<span class="line"><span style="color:#A6ACCD;">| employee_id    | </span><span style="color:#F78C6C;">int</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">6</span><span style="color:#A6ACCD;">)      | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   | PRI | </span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">       |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| first_name     | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">20</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| last_name      | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">25</span><span style="color:#A6ACCD;">) | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| email          | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">25</span><span style="color:#A6ACCD;">) | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   | UNI | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| phone_number   | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">20</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| hire_date      | </span><span style="color:#F78C6C;">date</span><span style="color:#A6ACCD;">        | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| job_id         | </span><span style="color:#F78C6C;">varchar</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">10</span><span style="color:#A6ACCD;">) | </span><span style="color:#F78C6C;">NO</span><span style="color:#A6ACCD;">   | MUL | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| salary         | double(</span><span style="color:#F78C6C;">8</span><span style="color:#A6ACCD;">,</span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| commission_pct | double(</span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">,</span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">) | YES  |     | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| manager_id     | </span><span style="color:#F78C6C;">int</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">6</span><span style="color:#A6ACCD;">)      | YES  | MUL | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#A6ACCD;">| department_id  | </span><span style="color:#F78C6C;">int</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">4</span><span style="color:#A6ACCD;">)      | YES  | MUL | </span><span style="color:#F78C6C;">NULL</span><span style="color:#A6ACCD;">    |       |</span></span>
<span class="line"><span style="color:#89DDFF;">+</span><span style="color:#676E95;font-style:italic;">----------------+-------------+------+-----+---------+-------+</span></span>
<span class="line"><span style="color:#F78C6C;">11</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">rows</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">in</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">set</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">.</span><span style="color:#F78C6C;">00</span><span style="color:#A6ACCD;"> sec)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div><p>其中，各个字段的含义分别解释如下：</p><ul><li>Field：表示字段名称。</li><li>Type：表示字段类型，这里 barcode、goodsname 是文本型的，price 是整数类型的。</li><li>Null：表示该列是否可以存储NULL值。</li><li>Key：表示该列是否已编制索引。PRI表示该列是表主键的一部分；UNI表示该列是UNIQUE索引的一部分；MUL表示在列中某个给定值允许出现多次。</li><li>Default：表示该列是否有默认值，如果有，那么值是多少。</li><li>Extra：表示可以获取与给定列有关的附加信息，例如AUTO_INCREMENT等。</li></ul><h2 id="_5-过滤数据" tabindex="-1">5. 过滤数据 <a class="header-anchor" href="#_5-过滤数据" aria-hidden="true">#</a></h2><ul><li>背景：</li></ul><p><img src="https://i0.hdslb.com/bfs/album/e3f7e8352bed8e7d1f2bbd689bf4627d6a078d35.png" alt=""></p><ul><li><p>语法：</p><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> 字段1,字段2</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> 表名</span></span>
<span class="line"><span style="color:#F78C6C;">WHERE</span><span style="color:#A6ACCD;"> 过滤条件</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><ul><li>使用WHERE 子句，将不满足条件的行过滤掉</li><li><strong>WHERE子句紧随 FROM子句</strong></li></ul></li><li><p>举例</p></li></ul><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> employee_id, last_name, job_id, department_id</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;">   employees</span></span>
<span class="line"><span style="color:#F78C6C;">WHERE</span><span style="color:#A6ACCD;">  department_id </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">90</span><span style="color:#A6ACCD;"> ;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="_6-课后练习" tabindex="-1">6.课后练习 <a class="header-anchor" href="#_6-课后练习" aria-hidden="true">#</a></h2><div class="language-sql line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">#第03章_基本的SELECT语句的课后练习</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;"># </span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;">.查询员工12个月的工资总和，并起别名为ANNUAL SALARY</span></span>
<span class="line"><span style="color:#A6ACCD;">#理解1：计算12月的基本工资</span></span>
<span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> employee_id,last_name,salary </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">12</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">ANNUAL SALARY</span><span style="color:#89DDFF;">&quot;</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">#理解2：计算12月的基本工资和奖金</span></span>
<span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> employee_id,last_name,salary </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">12</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> (</span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">IFNULL</span><span style="color:#A6ACCD;">(commission_pct,</span><span style="color:#F78C6C;">0</span><span style="color:#A6ACCD;">)) </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">ANNUAL SALARY</span><span style="color:#89DDFF;">&quot;</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;"># </span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">.查询employees表中去除重复的job_id以后的数据</span></span>
<span class="line"><span style="color:#F78C6C;">SELECT DISTINCT</span><span style="color:#A6ACCD;"> job_id</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;"># </span><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;">.查询工资大于12000的员工姓名和工资</span></span>
<span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> last_name,salary</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees</span></span>
<span class="line"><span style="color:#F78C6C;">WHERE</span><span style="color:#A6ACCD;"> salary </span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">12000</span><span style="color:#A6ACCD;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;"># </span><span style="color:#F78C6C;">4</span><span style="color:#A6ACCD;">.查询员工号为176的员工的姓名和部门号</span></span>
<span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> last_name,department_id</span></span>
<span class="line"><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> employees</span></span>
<span class="line"><span style="color:#F78C6C;">WHERE</span><span style="color:#A6ACCD;"> employee_id </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">176</span><span style="color:#A6ACCD;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;"># </span><span style="color:#F78C6C;">5</span><span style="color:#A6ACCD;">.显示表 departments 的结构，并查询其中的全部数据 </span></span>
<span class="line"><span style="color:#A6ACCD;">DESCRIBE departments;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#F78C6C;">SELECT</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">FROM</span><span style="color:#A6ACCD;"> departments;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br></div></div></div></div><!----></main><!--[--><!--]--><footer class="VPContentDocFooter" data-v-4fe9b7bd data-v-7f892bef><a class="prev-link" href="/mysql/02.html" data-v-7f892bef><span class="desc" data-v-7f892bef><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" viewbox="0 0 24 24" class="vt-link-icon" data-v-7f892bef><path d="M15,19c-0.3,0-0.5-0.1-0.7-0.3l-6-6c-0.4-0.4-0.4-1,0-1.4l6-6c0.4-0.4,1-0.4,1.4,0s0.4,1,0,1.4L10.4,12l5.3,5.3c0.4,0.4,0.4,1,0,1.4C15.5,18.9,15.3,19,15,19z"></path></svg> Previous</span><span class="title" data-v-7f892bef>02 【MySQL基本使用】</span></a><a class="next-link" href="/mysql/04.html" data-v-7f892bef><span class="desc" data-v-7f892bef>Next <svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" viewbox="0 0 24 24" class="vt-link-icon" data-v-7f892bef><path d="M9,19c-0.3,0-0.5-0.1-0.7-0.3c-0.4-0.4-0.4-1,0-1.4l5.3-5.3L8.3,6.7c-0.4-0.4-0.4-1,0-1.4s1-0.4,1.4,0l6,6c0.4,0.4,0.4,1,0,1.4l-6,6C9.5,18.9,9.3,19,9,19z"></path></svg></span><span class="title" data-v-7f892bef>04 【运算符】</span></a></footer></div></div></div></div><div class="visually-hidden" aria-live="polite" data-v-23306c50>03 【基本的SELECT语句】 has loaded</div></div></div>
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